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If you are in 'high fog' conditions with the fan turned off & the air intake turned to 'recirculate' [no fresh air coming in] then I have found that the screen fogs up with the speed of light. This is with no airflow across the screen.
Select 'fresh' air & blow it across the screen with a little heat added & the screen will clear. If you add aircon, it clears much faster, so I guess that the answer to your question would be 'Yes'

My 1000 sedan had an original defogger for the back window. It was a horizontal blower that simply blew air on the glass & it worked a treat. When the heater kicked in & some warm air was available to it, then it worked even better, so air movement across the glass is a must for fog clearing.
If the original outlet vents are sealed, & an alternative outlet is provided, then it should all be good, so long as the air can flow.

Of course, if the outlet vents are sealed & the car is parked in the sun, then any pressure rise from expanding air is simply vented through the intake side of the vent system, & even if this was sealed, it would bleed past the edge of the door glass via the woven material in the bailey channel, so windscreens would be safe.